Electrical connector

ABSTRACT

An electrical connector for transferring electromagnetic energy between a coaxial cable and a microstrip transmission system, each having a characteristic impedance of 50 ohms, with a minimal power loss due to a reflective discontinuity during the transfer. The electrical connector includes a central conductor and an outer conductor enclosing the central conductor, and has first, second, and transition sections each of which is designed to have a characteristics impedance of 50 ohms. The first and second sections are designed like sections of a coaxial cable to have a characteristic impedance of 50 ohms by the proper selection of the ratio of the diameters of the central and outer conductors. The central conductor of the first section has a small diameter, and the diameter of the outer conductor is a selected multiple of the small diameter to yield a characteristic impedance of 50 ohms. The second section has a large diameter central conductor, and the diameter of the outer conductor is the same selected multiple of the large diameter to yield a characteristic impedance of 50 ohms. The central conductor has an abrupt shoulder like transition between the small ad large diameters of the first and second sections. Similarly, the outer conductor has an abrupt shoulder like transition between the small and large diameters of the first and second sections. The transition section of the connector joins the first and second sections, and is constructed to have a short length of the small diameter of the central conductor enclosed within a short length of the large diameter of the outer conductor. The abrupt shoulder like transitions of the central and outer conductors define a large surface area between the central and outer conductors over a short conductive path. This large surface area introduces a significant capacitive reactance which is compensated for by positioning the short length of the small diameter central conductor in the large diameter outer conductor. This relative positioning introduces a significant inductive reactance into the transition section to compensate for the significant capacitive reactance introduced by the large surface area, and results in a characteristic impedance of 50 ohms in the transition section.

United States Patent 1 1 Brown 11] 3,725,829 1 1 Apr. 3, 1973 [54]ELECTRICAL CONNECTOR Deane David Brown, View, Calif.

[73] Assignee: Itek Corporation, Lexington, Mass. [22] Filed: July 14,1971 [21] Appl. No.: 162,432

[75] Inventor: Mountain 3,579,149 5/1971 Ramsey ..333/21 R 3,553,6071/1971 Lehrfeld ..333/84 M 2,938,175 5/1960 Sommers et al. ..333/97 X2/1965 Eason et al. ..333/21 R Primary Examiner-Rudolph V. RolinecAssistant Examiner-Saxfield Chatmon, Jr. Attorney-Homer 0. Blair et al.

57 ABSTRACT An electrical connector for'transferring electromagneticenergy between a coaxial cable and a microstrip transmission system,each having a characteristic impedance of 50 ohms, with a minimal powerloss due to a reflective discontinuity during the transfer. Theelectrical connector includes a central conductor and an odtemnductorenclosing the central conductor, and has first, second, and transitionsectionseach of which is designed to have a characteristics impedance of50 ohms. The first and second sections are designed like sections of acoaxial cable to have a characteristic impedance of 50 ohms by theproper selection of the ratio of the diameters of the central and outerconductors. The central conductor of the first section has a smalldiameter, and the diameter of the outer conductor is a selected multipleof the'small diameter to yield a characteristic impedance of 50 ohms.The second section has a large diameter central conductor, and thediameter of the outer conductor is the same selected multiple of thelarge diameter to yield a characteristic impedance of 50 ohms. Thecentral conductor has an abrupt shoulder like transition between thesmall ad large diameters of the first and second sections. Similarly,the outer conductor has an abrupt shoulder like transition between thesmall and large diameters of the first and second sections. Thetransition section of the connector joins the first and second sections,and is constructed to have a short length of thesmall diameter of thecentral conductor enclosed within a short length of the large diameterof the outer conductor. The abrupt shoulder like transitions of thecentral and outer conductors define a large surface area between thecentral and outer conductors over a short conductive path. This largesurface area introduces a significant capacitive reactance which iscompensated for by positioning the short length of the small diametercentral conductor in the large diameter outer conductor. This relativepositioning introduces a significant inductive reactance into thetransition section to co m pensate for the significant capacitive.

reactance introduced by the large surface area, and

' results in a characteristic impedance of 50 ohms in the transitionsection.

, 10 Claims, 3 Drawing Figures PATENTEDAPRB I973 3,7 5, 29

I NVENTOR 22 DEANE DA W0 BROWN ATTUR/VEY.

ELECTRICAL CONNECTOR BACKGROUND OF THE INVENTION coaxial mode of energypropagation and a strip mode 1 of energy propagation with a. relativelysmall power loss due to a reflective discontinuity at the transitionbetween the two modes of energy propagation.

When electromagnetic energy is transferred between coaxial and stripmodes of propagation, power losses due to reflective discontinuities atthe transition should be minimized. In general, a reflectivediscontinuity is formed by a mismatched impedance at the transition. Inthe prior art, these power losses have been minimized by utilizing thefollowing structure at the transition. The end section of the innerconductor of the coaxial cable is flattened, and the edges of thisflattened section are chamfered to reduce the width of the inner sectionto the width of the strip transmission line. The chamfered end of theinner conductor is soldered to the input or output lead of the stripconductor. Although this approach reduces the power loss at thetransition to a degree, there is still a reflective discontinuity whichcauses a partial loss of power during the energy transfer.

SUMMARY OF AN EMBODIMENT OF THE INVENTION In accordance with a preferredembodiment, an electrical connector is disclosed wherein electromagneticenergy is efficiently transferred between coaxial and strip modes ofpropagation with a minimal amount of power loss due to reflectivediscontinuities at the transition. The disclosed embodiment presents aconstant impedance to electromagnetic energy while changing the mode ofpropagation of the energy from coaxial to strip or vice versa.Electromagnetic energy has been transferred efficiently with thedisclosed embodiment over the frequency range of from DC to 12.5 Ghz.

In one embodiment, an electrical connector is disclosed for transferringelectromagnetic energy between a coaxial cable and a strip transmissionline with a minimal power loss due to an impedance mismatch at thetransfer. The electrical connector includes a central conductor and anouter conductor enclosing the central conductor and has first, second,and transition sections. The first and second sections are designed likesections of a coaxial cable according to art known techniques to havedesired characteristic impedances. The central conductor has arelatively thin first portion positioned in the first connector section,and a relatively thick second portion positioned in the second sectionof the connector. The outer connector has a first portion with arelatively small aperture which encloses the first thin portion of thecentral conductor in the first connector section and a second portionwith a relatively large aperture which encloses the second thick portionof the central conductor in the second section of the electricalconnector. The transition section of the connector joins the first andsecond sections, and is constructed to have short length of therelatively thin first portion of the inner conductor enclosed within therelatively large aperture of the outer'conductor. The transitionsbetween the first and second portions of each of the central and outerconductors define a large surface area between the central and outerconductor over a short conductive path. This large surface areaintroduces a significant capacitive reactance which is compensated forby positioning the short length of the thin central conductor in thelarge aperture of the outer conductor. This relative positioningintroduces a significant inductive reactance into the transition sectionto combine with the significant capacitive reactance introduced by thelarge surface area, and results in a desired characteristic impedance inthe transition section.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF A PREFERREDEMBODIMENT FIG. 1 illustrates a microstrip circuit 10 which is 2 formedon a ceramic substrate 11. The circuit has an input lead 12 and anoutput lead 14. The particular circuit illustrated in FIG. 1 is a bandpass interdigital filter circuit, although this particular circuit isshown merely for illustrative purposes, and any microstrip circuit couldbe utilized with this invention. 7

FIG. 2 illustrates a housing 20 which encloses the microstrip circuit,and protects the circuit from physical damage and electrical noise. Thehousing has a main body 22 with a central aperture 28 formed therein.Central aperture 28 has a peripheral ledge 30 upon which the circuit ispositioned. Thehousing has input and output contacts 32 which transferelectrical energy to the input and output leads of the microstripcircuit. Housing 22 has threaded connectors 24 and 26 for connectionwith input and output coaxial cables. The microstrip circuit is placedwithin the central aperture 28 of the housing, and cover 34 is placed onthe housing and secured by screws 36. An elastic conductive sheet 38 ispositioned below the cover 34 to compliantly hold the circuit 10 withinthe housing 20. The sheet 32 forms an electrical contact with thecircuit and holds the circuit within the housing without physicallystressing the ceramic substrate. This arrangement lessens thepossibility of cracking the ceramic substrate when cover 34 is securedin place with screws 36.

FIG. 3 illustrates a partially sectioned view of one embodiment of anelectrical connector which is utilized in the housing to transferelectrical energy between coaxial and strip modes of propagation.Conductor 40 of connector 26 is threaded at 42 to receive a coaxialcable in a conventional manner while making an electrical contact withthe outer conductor of the coaxial cable. The inner lead of the coaxialcable makes an electrical contact with a central conductor 44 of theelectrical connector which leads to a contact of the microstrip circuit.

Conductor. 40 is electrically grounded to housing 22 to form twosections of an outer conductor of the electrical connector. There is anabrupt shoulder like transition 45 between the inner diameter 41 ofconductor 40 and the inner diameter 48 of aperture 47 in housing 22. Thesignificance of this structure will be explained later. 7

Central conductor 44 has a first smaller cylindrically shaped section 54and a second larger cylindrically shaped section 58. There is an abruptshoulder like transition 55 between section 54, which has a relativelysmall diameter 56, and cylindrical section 58, which has a relativelylarge diameter 60. The significance of this structure will be explainedlater. Central connector 44 has a cantilevered tab portion 32 at one endfor making an electrical contact with the microstrip circuit. Tab 32 isbent slightly relative to the central con- 7 ductor to form a tensionedcantilever-type contact with the microstrip circuit in the housing. Thesection 58 has a hole 64 formed axially therein for receipt of thecentral conductor of the coaxial cable. Two slits 66, are out along andon opposite sides of hole 64. The end section 58 is crimped asillustrated along each of the slits 66. This crimping allows the centralconductor of the coaxial cable to be resiliently force fitted into hole64.

Dielectric 46 insulates and relatively positions the central and outerconductors of the electrical connector. Dielectric 46 has a firstsmaller cylindrically shaped section 68 and a second largercylindrically shaped section 72. A first cylindrically shaped aperture70 is formed axially along the entire length of first sec-, tion 68 andpartially into second section 72 for a distance 49. A secondcylindrically shaped aperture 74 is formed axially along the remainderof the length of second section 72.

An aperture 50 is illustrated which extends widthwise across theassembly of outer conductor 42, dielectric member 46, and centralconductor 44. This aperture has been provided so that it may be filledwith a potting or glue material to permanently attach the componentsinto one unitary assembly.

The significance of the dimensions which were mentioned previously willnow be explained. ln a typical microstrip circuit the characteristicimpedance of the circuit is 50 ohms. The characteristic impedance of acoaxial cable carrying electrical energy to and from the microstripcircuit is also normally selected to be 50 ohms. The dimensions of theelectrical connector are selected to transfer electrical energy betweencoaxial and strip modes of propagation with a characteristic impedanceof 50 ohms. A different impedance would result in a reflectivediscontinuity to electrical energy at the connector.

The dimensions for the electrical connector are selected as follows. Thewidth ofv the input and output cording to the ratio of the diameters ofthe inner and ductor and the inner diameter 52 of the outer conductor 40are also selected to yield a characteristic impedance of ohms. It shouldbe noted that the ratio of these two diameters, for reasons explainedabove, is the same as the ratio of the two diameters in the firstsection of the connector. I 1

The first and second sections of the connector must also be joined witha characteristic impedance of 50 ohms. This joinder is accomplishedacross the transition section 49 wherein a short length of the smallerdiameter central conductor is located within a short length. the largerdiameter outer conductor. This section of the connector operates asfollows. As is well known in the art, characteristic impedance is acombination of capacitive and inductive reactance. The abrupt shoulderlike transitions 45 and of the inner and outer conductors, between eachof the two sections thereof, define a large surface area between theconductors over a short conductive path. This large surface areaintroduces a significant capacitive reactance between the inner andouter conductors. This significant capacitive reactance is compensatedforby positioning a short length of the smaller diameter centralconductor within a short length 'of the larger diameter outer conductor.This combination introduces a significant inductive reactance. This:significant inductive reactance compensatesfor the significantcapacitivereactance introduced by the large surface area, and results in acharacteristic. impedance of 50 ohms in section 49. An optimum length ofthe transition section may be determined imperically by testingdifferent lengths of the transition section for power transfer orpower'reflectance. One'instrument that might be used to make such ameasurement is a time domain reflectometer.

Thus, the electrical connector is formed of three sections, each ofwhich is designed to have a characteristic impedance of 50 ohms. Thisdesign presents a relatively small reflective discontinuity toelectrical energy flowing through the electrical connector.

One embodiment of the electrical connector was built according to thefollowing specifications. The housing 22 and conductor 40 wereconstructed of gold plated stainless steel. The central conductor 44 wasconstructed of beryllium copper, with cantilevered tab 32 being goldplated. The dielectric 46 was formed from polytetrafluoroethyleneplastic rod. The conductive sheet 38 was made from silicon rubberimpregnated with silver. Binding hole 50 was potted with The principleexplained above of introducing inductive reactance to compensate for thelarge capacitive reactance introduced by the abrupt shoulder likechanges may be extended as follows. if the change in diameter betweenthe several sections of the connector is very large, the connector maybe designed with several steps of an abrupt reduction in diameter withseveral introductions of compensating inductive reactance.

While several embodiments have been described, the teachings of thisinvention will suggest many other embodiments to those skilled in theart.

Iclaim:

1. Apparatus for transferring electromagnetic energy between a coaxialcable and a strip transmission line with a relatively small power lossdue to a reflective discontinuity comprising:

a. said strip transmission line having first and second conductiveelements;

b. said coaxial cable having a central conductor and an outer conductor;

c. a connector having a first conductor for connecting said centralconductor of the coaxial cable with said first conductive element of thestrip transmission line, said first conductor having a relatively thinfirst section, and a relatively thick second section adjoining saidfirst section with the joinderof said first and second sections of saidfirst conductor defining a large surface area over a short conductivepath which introduces a capacitive reactance;

d. said connector having a second conductor for connecting said outerconductor of the coaxial cable with said second conductive element ofthe strip transmission line, said second conductor having a firstsection with a relatively small aperture and a second section, adjoiningsaid first section, with a relatively large aperture with the joinder ofsaid first and second sections of said second conductor defining a largesurface area over a short conductive path which introduces a capacitivereactance; and

e. means for supporting said second section of said first conductorwithin said second section of said second conductor, and for supportingsaid first section of the first conductor within said first section ofsaid second conductor and partially within said second sectionof saidsecond conductor to introduce an inductive reactance to compensate forsaid capacitive reactances, whereby the relatively thin first section ofsaid first conductor being partially positioned within the relativelylarge aperture of said second conductor results in a desired impedanceto electromagnetic energy being transferred between said striptransmission line and said coaxial cable.

2. Apparatus as set forth in claim 1 wherein said first conductor has anabrupt, shoulder like transition between said relatively thin firstsection and said relatively thick second section, and said secondconductor has an abrupt, shoulder like transition between said small andlarge apertures.

3. Apparatus as set forth in claim 1 wherein said first and secondsections of said first conductor each have cylindrical shapes andsaid'small and large apertures of said second conductor each havecylindrical shapes."

4. Apparatus as set forth in claim 1 wherein only a small portion ofsaid first section of said first conductor is located within said secondsection of said second conductor.

5. Apparatus as set forth in claim 1 wherein the characteristicimpedance of said coaxial cable is 50 ohms, the characteristic impedanceof said strip transmission line is 50 ohms, and the characteristicimpedance of said connector is 50 ohms.

6. Apparatus as set forth in claim 1 wherein said rela tively thin firstsection of said first conductor includes a cantilevered sectionprotruding from the end of said first section for making an electricalcontact with the strip transmission line.

7. Apparatus as set forth in claim 1 wherein said strip transmissionline is a microstrip slab, and is supported within a housing means.

8. Apparatus as set forth in claim 1 wherein:

a. said first and second sections of said first conductor each havecylindrical shapes, and said small and large apertures of said secondconductor each have cylindrical shapes;

b. said first conductor has an abrupt, shoulder like transition betweensaid relatively thin first cylindrical section and said relatively thicksecond cylindrical section; and

c. said second conductor has an abrupt shoulder like transition betweensaid small and large cylindrical apertures.

9. Apparatus as set forth in claim 8 wherein the characteristicimpedance of said coaxial cable is '50 ohms, the characteristicimpedance of said strip transmission line is 50 ohms, and thecharacteristic impedance of said connector is 50 ohms.

10. Apparatus as set forth in claim 9 wherein said strip transmissionline is a microstrip slab, and is supported within a housing means.

1. Apparatus for transferring electromagnetic energy between a coaxialcable and a strip transmission line with a relatively small power lossdue to a reflective discontinuity comprising: a. said strip transmissionline having first and second conductive elements; b. said coaxial cablehaving a central conductor and an outer conductor; c. a connector havinga first conductor for connecting said central conductor of the coaxialcable with said first conductive element of the strip transmission line,said first conductor having a relatively thin first section, and arelatively thick second section adjoining said first section with thejoinder of said first and second sections of said first conductordefining a large surface area over a short conductive path whichintroduces a capacitive reactance; d. said connector having a secondconductor for connecting said outer conductor of the coaxial cable withsaid second conductive element of the strip transmission line, saidsecond conductor having a first section with a relatively small apertureand a second section, adjoining said first section, with a relativelylarge aperture with the joinder of said first and second sections ofsaid second conductor defining a large surface area over a shortconductive path which introduces a capacitive reactance; and e. meansfor supporting said second section of said first conductor within saidsecond section of said second conductor, and for supporting said firstsection of the first conductor within said first section of said secondconductor and partially within said second section of said secondconductor to introduce an inductive reactance to compensate for saidcapacitive reactances, whereby the relatively thin first section of saidfirst conductor being partially positioned within the relatively largeaperture of said second conductor results in a desired impedance toelectromagnetic energy being transferred between said strip transmissionline and said coaxial cable.
 2. Apparatus as set forth in claim 1wherein said first conductor has an abrupt, shoulder like transitionbetween said relatively thin first section and said relatively thicksecond section, and said second conductor has an abrupt, shoulder liketransition between said small and large apertures.
 3. Apparatus as setforth in claim 1 wherein said first and second sections of said firstconductor each have cylindrical shapes and said small and largeapertures of said second conductor each have cylindrical shapes. 4.Apparatus as set forth in claim 1 wherein only a small portion of saidfirst section of said first conductor is located within said secondsection of said second conductor.
 5. Apparatus as set forth in claim 1wherein the characteristic impedance of said coaxial cable is 50 ohms,the characteristic impedance of said strip transmission line is 50 ohms,and the characteristic impedance of said connector is 50 ohms. 6.Apparatus as set forth in claim 1 wherein said relatively thin firstsection of said first conductor includes a cantilevered sectionprotruding from the end of said first section for making an electricalcontact with the strip transmission line.
 7. Apparatus as set forth inclaim 1 wherein said strip transmission line is a microstrip slab, andis supported within a housing means.
 8. Apparatus as set forth in claim1 wherein: a. said first and second sections of said first conductoreach have cylindrical shapes, and said small and large apertures of saidsecond conductor each have cylindrical shapes; b. said first conductorhas an abrupt, shoulder like transition between said relatively thinfirst cylindrical section and said relatively thick second cylindricalsection; and c. said second conductor has an abrupt shoulder liketransition between said small and large cylindrical apertures. 9.Apparatus as set forth in claim 8 wherein the characteristic impedanceof said coaxial cable is 50 ohms, the characteristic impedance of saidstrip transmission line is 50 ohms, and the characteristic impedance ofsaid connector is 50 ohms.
 10. Apparatus as set forth in claim 9 whereinsaid strip transmission line is a microstrip slab, and is supportedwithin a housing means.